Airborne measurements of the electromagnetic bias

Abstract

The sea-level estimation of satellite altimetry is affected by a sea-state related error, the so called electromagnetic (EM) bias. The ocean-wave troughs reflect the radar signals stronger than the crests, which then biases the altimeter mean sea level measurements towards the troughs.^ In order to establish an EM bias correction algorithm based on those parameters available from the satellite platform, namely the significant wave height and the normalized radar cross section (NRCS), the University of Massachusetts (UMass) collaborated with the Goddard Space Flight Center of NASA in four airborne experiments: EM Bias Experiment (1989), Shelf Edge Exchange Processes Experiment (SEEP 1989), Surface Wave Dynamics Experiment (SWADE 1991), and Southern Ocean Wave Experiment (SOWEX 1992). All four employed the 5.3 and 13.6 GHz frequencies currently being used by the TOPEX/POSEIDON satellite altimeters. The 5.3/13.6 GHz dual-frequency, nadir-looking scatterometer of UMass measured ocean surface NRCS by spatially resolving wave troughs and crests, while the Airborne Oceanographic Lidar and the 36 GHz Scanning Radar Altimeter of NASA profiled waves, and provided directional wave spectra. Sea state conditions were also derived from buoy recordings, primarily during the SWADE missions.^ The first three experiments provided data for the prelaunch EM bias correction curves of the TOPEX/POSEIDON altimeters, while SOWEX complemented the earlier data set with high-wind, high-wave-height measurements. The evaluation of the TOPEX/POSEIDON performance has found that the prelaunch curves slightly overestimate the bias at both frequencies, meanwhile they predict the bias difference at the two frequencies correctly. On-orbit data analysis, as well as the observations from SOWEX suggest that both the 5.3 and 13.6 GHz biases decrease for high winds, which fact was taken into account during the revision of the EM bias correction algorithms. ^